1. Field of the Invention
The present invention relates to a touch panel mainly used for operations of a variety of electronic instruments.
2. Description of the Related Art
Recently, the functionality and diversity of a variety of electronic instruments such as portable phones and car navigation systems are increasing. With these increases, instruments equipped with light-transparent touch panels on the front surfaces of display devices such as liquid display devices are increasing in number. A user switches a variety of functions of the instruments by pressing a touch panel with his or her finger, a pen or the like, while viewing a display of the display device on the back surface through the touch panel.
Such a conventional touch panel is described with reference to FIGS. 4 through 6. It is to be noted that a size is partially enlarged in each of the drawings for the sake of facilitating understanding of the configuration. FIG. 4 shows a sectional view of a conventional touch panel, and FIGS. 5 and 6 are plan views of an upper substrate and a lower substrate of this touch panel, respectively. This touch panel has upper substrate 1, lower substrate 2, upper conductive layer 3, lower conductive layer 4, and wiring substrate 11. It is to be noted that both FIGS. 5 and 6 show the upper surface of wiring substrate 11.
Upper substrate 1 has the shape of a film or a thin plate, and is light-transparent. Lower substrate 2 is also light-transparent. Upper conductive layer 3 is substantially rectangular, formed of indium tin oxide or the like, light-transparent, and formed on the lower surface of upper substrate 1 by etching or the like. In the same manner, lower conductive layer 4 is formed on the upper surface of lower substrate 2.
At the front and rear ends of upper conductive layer 3, front electrode 5 and rear electrode 6 are formed of silver or the like. Further, left electrode 7 and right electrode 8 are formed at the left and right ends of lower conductive layer 4. These electrodes extend to the peripheries of upper conductive layer 3 and lower conductive layer 4, and derivation sections 5A, 6A, 7A, 8A are provided at the front end of the touch panel.
On the upper surface of lower conductive layer 4, a plurality of dot spacers (not shown) are formed of an insulating resin at prescribed intervals. Furthermore, spacer 9 having a frame shape is provided at the peripheral inner edge of the upper surface of lower substrate 2. Adhesive layer 10 is applied to the upper surface of spacer 9. The periphery of upper substrate 1 and the periphery of lower substrate 2 are attached to each other by adhesive layer 10. In such a manner, upper conductive layer 3 and lower conductive layer 4 are opposed to each other with a prescribed space therebetween.
The rear end of wiring substrate 11 having a film shape is sandwiched between the front end of upper substrate 1 and the front end of lower substrate 2. Wiring patterns 12, 13 are respectively formed on the upper and lower surfaces of wiring substrate 11. The rear ends of wiring patterns 12, 13 are attached to each of derivation sections 5A, 6A, 7A, 8A by an anisotropic conductive adhesive (not shown) obtained by dispersing conductive particles within a synthetic resin, or the like. In such a manner, the touch panel is configured.
This touch panel is arranged on the front surface of a display device such as a liquid crystal display device, and mounted in an electronic instrument. The front ends of wiring patterns 12, 13 of wiring substrate 11 are electrically connected to an electronic circuit (not shown) of the instrument by means of connectors, soldering, or the like.
In the above configuration, when the user presses the upper surface of upper substrate 1 with his or her finger, a pen, or the like in accordance with a display of the display device on the rear surface of the touch panel, upper substrate 1 is bent, and upper conductive layer 3 comes into contact with lower conductive layer 4 at the pressed place. Subsequently, a voltage is sequentially applied from the electronic circuit to the front and rear ends of upper conductive layer 3 via front electrode 5 and rear electrode 6, and to the left and right ends of lower conductive layer 4 via left electrode 7 and right electrode 8, through wiring patterns 12, 13 of wiring substrate 11. With a ratio of these voltages, the electronic circuit detects the pressed place, and a variety of functions of the instrument are switched.
Namely, for example, with a plurality of menus in the state of being displayed on the display device of the rear surface of the touch panel, the user presses the upper surface of upper substrate 1 on a desired menu. The electronic circuit then detects the pressed (operated) position through wiring patterns 12, 13 of wiring substrate 11. With such an operation, the user can select a desired menu from the plurality of menus.
In the case of detecting the operated position in the manner as thus described, the electronic circuit typically applies a voltage of approximately DC 3 to 5 V between the front and rear ends of upper conductive layer 3 and between the left and right ends of lower conductive layer 4. However, at the time of supplying a current to front electrode 5, rear electrode 6, left electrode 7 and right electrode 8, especially in the electrode with long extended portion, voltage drop occurs due to the resistance thereof, and hence differences in voltage value occur among each electrode.
This situation is described with reference to rear electrode 6 shown in FIG. 5. Rear electrode 6 is extended from the rear end of upper conductive layer 3 to the left end of upper substrate 1, and further extended to the front end of upper substrate 1. Therefore, rear electrode 6 is longer than the other electrodes. For example, voltage drop at a point A near derivation section 6A is 0.1 V. However, voltage drop is 0.15 V at a point B and 0.2 V at a point C. In other words, voltage drop is larger as the point is more distant from derivation section 6A. When a difference in voltage value occurs in a long electrode due to voltage drop, an error occurs in detection of an operated position.